06/14/2026
🐛Forest Tent Caterpillars: have you seen them in your trees? Have a read!
Since everyone is posting pictures all over Facebook of the caterpillars on trees (and many concerned for the health of their trees) I'm going to share a little bit of info about them!
1) We have two main species of tent caterpillar in Alberta: forest and western. Tent caterpillars go through cycles of around ten years (8 to 10 for western and 10 to 12 for forest). They will boom like this for a couple years, then be almost non-existent until the next boom. They will not continue like this indefinitely, and these booms are an essential part of the natural system of the ecosystem.
2) They will not kill your trees. If your tree dies after a tent caterpillar infestation, then your tree was already unhealthy (unhealthy trees do not always appear unhealthy). Healthy trees will look ugly for the season, but will be fine.
3) Tent caterpillars are native to Alberta and their cycles (and booms) are very important for the ecosystem. They feed countless other critters including migratory birds, and allow for bird populations to have a very successful breeding year, sometimes able to have multiple clutches of eggs in one season and more young surviving to maturity due to the abundance of easy food.
4) They produce a LOT of frass (p**p) which provides vital contributions to the nutrient cycle. Not only do the nutrients in frass allow the tree to gain extra energy for regrowing the lost leaves, but there's more nutrients being added than the tree itself needs, so the soil surrounding host trees becomes quite rich following these booms.
5) In the forested areas, these cycles are actually a necessary part of the ecosystem. They clear out foliage for a season, allowing openings in the canopies that let in sunlight to the ground level, which allows other forest cycles to continue. They also kill off diseased and infected trees, allowing for necessary decomp that also adds nutrients into the ecosystem, and makes room for healthy saplings to fill the void of the sick trees.
6) This appears to be a parasitic relationship, and to the individual tree, it may be. However, it's actually more of a symbiotic relationship, and the health of the tree population as a whole benefits from tent caterpillar cycles.
7) Likely the most important fact for people strictly concerned for their trees is that removing the caterpillars from trees after they've began eating leaves can be worse for your trees. Tent caterpillars will eliminate all their waste prior to moving onto the next tree, so they can travel more quickly. So the biggest nutrient dump happens right before they leave. If you kill them all mid way through, you are depriving your tree of all that nutrient rich frass that they need to regrow their leaves. If the caterpillars have already done a number to the foliage and you remove them, it can actually take longer for your tree to recover, and leaves them more susceptible to diseases and infection.
I can't stop anyone from hating on them, and the truth is, some people WILL lose trees after this and blame the caterpillars for taking out a tree that was, unbeknownst to them, going to die in the next few years regardless. But hopefully some people will read this from a biological perspective and think just a little differently about this amazing natural event that intertwines into so many other aspects of the environment!
